1. Field of Invention
Embodiments relate to electronic amplifier output signal power detection and control, particularly for radio frequency (RF) signals output from one or more final stage amplifying transistors.
2. Related Art
Many electronic circuit applications require that output power of an amplifier be detected and controlled to maintain a specified level. Such an output power level may be required to change with time. In a Global System for Mobile communications (GSM) cellular telephone handset, for example, the final stage radio frequency (RF) amplifier outputs a signal to the antenna, and the power level of this signal must be controlled to maintain one or more precise levels. Therefore, it is important to be able to determine amplifier output power level and to control that power level.
The power of a signal output from a Class C or similar transistor amplifier with non-continuous conduction corresponds to the collector current. In such amplifiers, two or more transistors may be connected in parallel so that the signal output from the transistors corresponds to the combined collector currents. As is well-known for a bipolar junction transistor that operates in the cutoff or forward active region, the base current controls the collector current, and the collector current depends on the base-emitter voltage. Thus the base-emitter voltage is strongly correlated with the power of the signal output from the amplifier. It is therefore known that a desired output power level of an amplifying transistor can be maintained by sensing the output power and then adjusting the base bias voltage of the transistor to maintain the desired output power level.
One well-known method of sensing the power of an output signal from a transistor amplifier is to use a directional coupler. Such a directional coupler senses either forward or reflected signal power in a transmission line. A disadvantage of directional couplers, however, is that some coupling loss exists as the output signal is sampled. In addition, directional couplers are typically large, a condition which prevents the coupler from being on the same integrated circuit as the amplifier. Thus, additional area is required in an electronic device (e.g., a cellular telephone handset) to accommodate the coupler. Furthermore, adding an off-chip coupler to an electronic device increases the cost of the device.
What is desired is a way of sensing and controlling amplifier output signal power without significant loss, with little expense, and with little use of device area.